Hyperfocal with Leica M8

[Peter Branch]

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This is not correct. Rays from a point in the subject form a cone with the basis = the aperture (strictly, its projection on the main optical plane) and its apex on the plane of best focus, hopefully on the sensor. So the rays converge in front of the focal plane, and diverge behind it. Depth of field depends on how fast they converge and diverge. This is given by the apex angle of the cone. This is *exclusively* dependent on a) the diameter of the cone's base, i.e. the diaphragm opening, and the distance from this base to the sensor/apex. Simple geometry. The size of the sensor has absolutely nothing to do with this. It cannot change the cone's geometry. Ergo, sensor size does not affect d.o.f. Q.E.D. Sorry, I can't make it simpler than that, because it isn't simpler!

 

The old man from the Age of Euclidean Geometry

 

Lars, if only it were that simple!

 

If you really want to get into this subject you need to take account of the Entry Pupil / Exit Pupil ratio. This is why Leica publish separate Depth of Field tables for each of their lenses and it is instructive to look at the small but real differences between M lenses of the same focal length and more particularly compare M lenses with their R equivalents. Retro focus has a significant impact.

 

In general terms my explanation is correct, but it does assume that we are dealing with “simple” lenses – that I grant you.

 

My key point is that in many of the explanations in this thread there is confusion being caused by attributing one aspect of this complex subject to some factor which has nothing to do with it. The geometric arguments relating to Circle of Confusion, Field of View and the related factors of Image Size, Magnification, Minimum Viewing Distance and Visual Acuity, (The last being surprisingly the one about which experts tend to agree!) are sound, but, as Sean has pointed out there are just too many assumptions and variables to make sense of all this in the real world. His advice to go out and learn the craft of photography by doing it I heartily endorse.

 

I have read, and generally understood, most of the papers referred to in this thread but am still not convinced that we have a satisfactory explanation and guide to Depth of Field and all the related topics, such as the Hyperfocal Distance, that works as well in the Digital domain as the old “rules” worked with film. My experience is that despite the theoretical advantages of the smaller sensor and the shorted focal lengths the visual effect is more critical and Depth of Field seems actually to be diminished. But to repeat this is not due to the geometric effects; its something else that nobody has yet explained to my satisfaction. I’ve learned to be more careful with almost all aspects of focus, lens selection, aperture etc with my M8.

[rosuna]

Does crop affect the DoF?

 

It depends on how you make the comparison. Keeping the distance to the subject and the print size unchanged, and croppig the sensor or picture, the DoF is reduced, because the circle of confusion is reduced. If you change the distance to the subject in order to have the same angle of view on the full frame and the cropped-sensor camera, then the DoF can be wider in the print from the cropped-sensor camera.

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[jaapv]

Lars, if only it were that simple!

 

If you really want to get into this subject you need to take account of the Entry Pupil / Exit Pupil ratio. This is why Leica publish separate Depth of Field tables for each of their lenses and it is instructive to look at the small but real differences between M lenses of the same focal length and more particularly compare M lenses with their R equivalents. Retro focus has a significant impact.

 

In general terms my explanation is correct, but it does assume that we are dealing with “simple” lenses – that I grant you.

 

My key point is that in many of the explanations in this thread there is confusion being caused by attributing one aspect of this complex subject to some factor which has nothing to do with it. The geometric arguments relating to Circle of Confusion, Field of View and the related factors of Image Size, Magnification, Minimum Viewing Distance and Visual Acuity, (The last being surprisingly the one about which experts tend to agree!) are sound, but, as Sean has pointed out there are just too many assumptions and variables to make sense of all this in the real world. His advice to go out and learn the craft of photography by doing it I heartily endorse.

 

I have read, and generally understood, most of the papers referred to in this thread but am still not convinced that we have a satisfactory explanation and guide to Depth of Field and all the related topics, such as the Hyperfocal Distance, that works as well in the Digital domain as the old “rules” worked with film. My experience is that despite the theoretical advantages of the smaller sensor and the shorted focal lengths the visual effect is more critical and Depth of Field seems actually to be diminished. But to repeat this is not due to the geometric effects; its something else that nobody has yet explained to my satisfaction. I’ve learned to be more careful with almost all aspects of focus, lens selection, aperture etc with my M8.

 

 

Peter, the effect you mention is real and has been explained in several publications, of which LFI, I think 2/2007 (I would have to find it to be sure) is the latest.

Film has an appreciable thickness of emulsion, it varies per film, but for the sake of the argument it can be put at 0.010 to 0.020 mm. This makes the cause of DOF, the Circle of Confusion,at a theoretical ( but too large) value of 0.023 on a 1.3 crop medium, a three-dimensional phenomen, with the circles getting a skewed cone shape to the edges of the image. In the emulsion we see diffraction and reflections etc. This makes the DOF in film soft, not even considering that film is not flat either. A sensor, on the other hand behaves like the perfect plane, making the COC much more defined and the DOF more steep. This gives the impression of a smaller DOF, as this, as I once said in a similar discussion, is in the eye of the beholder.

[Jamie Roberts]

I think the conclusion of the Luminous Landscape article (and there are many more) on lens equivalencies is pertinent here.

 

Yes, the author begins by saying exactly what Andy and Lars have said; except the size of the sensor *does in fact* change the equation due to different COC calculations, as Howard C. has been pointing out.

 

The conclusions have nothing to do with film depth; everything to do with sensor size and equivalence:

 

"3. The smallest sensor we considered (denoted 1/2.5”) achieves maximum useful DoF at an f-number of N=3.5 while the full frame 35 mm sensor requires N=21 for a factor of 36 difference in transmitted light. If the full frame sensor gives the same signal/noise ratio (S/N) at ISO sensitivity 1600 as the small sensor does at ISO 80, the small sensor can still use a higher shutter speed. A PS sensor that could give low noise at ISO 800 or 1600 would appear to have a real advantage over FF sensors.

4. If maximizing the DoF is not the aim, larger sensors clearly win because of their ISO sensitivity advantage. A fast lens (N=1.4) with a full frame detector is impossible to match with the small sensor. Probably N=1 is the maximum aperture we can expect with a small sensor, and no company at present even offers N=2. The take home lesson is that small sensors should be coupled with large aperture lenses, i.e. small N values.

 

Luminous landscape "

 

This article--and the follow-on by Nathan Myhvorld where he actually talks about diffraction sensor limits--completely and totally agree with my empirical experience using both cropped bodies (down to a 10d and G7) and full-frame bodies (1ds2)

 

In other words, the sensor does affect the practical DOF.

 

Case in point: I know a very well esteemed photographer who shoots his 85 1.2L exclusively on a 1.6 crop body, precisely so he will have effectively more depth of field but retain the 1.2 light-gathering characteristics.

 

Now, what Andy and Lars would tell me is that if we both shot at the same distance, the 1ds2 full-frame would have exactly the same DOF effects as the 1.6 cropped body, the difference being the 1ds2 would have more of the context as an 85 (not 135 effective) lens.

 

Then they might go on to say (if I understand this correctly) that if I move up to catch the same crop I have thus effectively reduced the DOF such that I would have to stop down quite a bit to retain critical focus.

 

I understand all that. But what I understand from the LuLa article is that this is simply not the case; that if we stood in the same place, the cropped shot would still have more DOF.

 

Am I reading the article incorrectly?

 

(oh and Bill--if I understand correctly the article also mentions that the sensor plays a role in diffraction limiting of resolution beyond lens diffraction; by stopping down so much on that CV lens you might gain DOF but lose much resolution between the lens and the sensor)..

[cme4brain]

Jamie, my dof calculation was as follows:

 

My 35mm becomes a 47mm (or so). That's a longer lens. A 47 has a narrower dof than a 35, other things being equal.

 

If I get you, you're saying that my 47 is really as 35 and enjoys the 35's dof.

 

Which is the operative process?

 

 

 

As to the CV15, I've tried with and without a tripod. It's definitely NOT Leica glass (not a surprise at 10% of the cost). Perhaps I don't have the screw mount tight enuf. At f11, I can get a satisfactory image. At 5.6 or 8, I don't like what I get. This may be due to the fact that the out-of-focus part of the image sucks.

 

If you look at the image above, the grass in the background, where the pieces of grass in the foreground are in focus, doesn't resemble the background images I have come to expect from Leica glass.

 

I need a wide lens. Not finding a WATE to my specifications, I bot the CV15. As soon as Leica makes a 15mm f1.4, I'm giving the CV15 away (don't all shout at once; the candidate has been chosen) or selling it. If they goof off too long, I'll go for the Zeiss.

 

I know that I am an amateur, but my CV 15 images on my M8, when guess focused well, are razor sharp; I get the same results as Sean Reid does in his website review of superwides. Maybe your elements are decentered, or something else is wrong.

[adan]

Lars: Without quoting the whole thing, your Euclidian Geometry essay is correct - as far as it goes.

 

However, it does not go far enough, because it stops at the image as projected on the sensor or film.

 

And we never view the final image at that size. We always enlarge it. Even looking at 35mm film contact sheets, one uses a loupe of 8x or 10x to judge apparent sharpness (and DOF is nothing if not APPARENT sharpness)

 

Any discussion of DOF must go beyond the captured image to include enlargement factors.

 

Technically, in fact what you describe - the cone of light as it hits the image plane/sensor/film - is referred to as Depth of Focus: Depth of focus - Wikipedia, the free encyclopedia

 

It is NOT directly equatable to Depth of Field - one must add the additional factor of final enlargement of the captured image (2x, 10x, 20x) to calculate that.

[gesper]

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I know that I am an amateur, but my CV 15 images on my M8, when guess focused well, are razor sharp; I get the same results as Sean Reid does in his website review of superwides. Maybe your elements are decentered, or something else is wrong.

 

Same here.

[Peter Branch]

Peter, the effect you mention is real and has been explained in several publications, of which LFI, I think 2/2007 (I would have to find it to be sure) is the latest.

Film has an appreciable thickness of emulsion, it varies per film, but for the sake of the argument it can be put at 0.010 to 0.020 mm. This makes the cause of DOF, the Circle of Confusion,at a theoretical ( but too large) value of 0.023 on a 1.3 crop medium, a three-dimensional phenomen, with the circles getting a skewed cone shape to the edges of the image. In the emulsion we see diffraction and reflections etc. This makes the DOF in film soft, not even considering that film is not flat either. A sensor, on the other hand behaves like the perfect plane, making the COC much more defined and the DOF more steep. This gives the impression of a smaller DOF, as this, as I once said in a similar discussion, is in the eye of the beholder.

 

Jaap, I think that the subjective impression of sharpness has a great deal to do with the perceived Depth of Field, (“DoF”). If you think about it the issue with film, (about which I know a little having worked for Kodak for over 40 years), is that the effects of dispersion, (aka scatter), and internal reflection, (aka halation), increase the size of the image point, just as you say – but – this will have the effect of making the image more “out of focus” and thus, in principle, reduce the DoF. Some years ago I took hundreds of pictures on Tech Pan Film which had near microfilm resolution etc. The visual effect was a small, but noticeable, increase in perceived DoF compared with films like T-Max 100. Those pictures were taken mainly with a 50 mm Summicron-R which is optically almost the same as the current 50mm f/2 for the M.

 

It is my belief that as well as the geometric effects which can be calculated with exquisite precision if one defines a set of assumptions and takes account of the parameters for the specific lens being studied there is a relative effect based on visual perception which has nothing to do with the geometry. For some reason the images from the digital sensor, whatever the theory, appear sharper at the point of true focus than anything that is possible with film, except under laboratory conditions. If this is correct it is not difficult to understand that the apparent DoF is reduced because the eye sees even the very nearly in focus parts of the image as being obviously out of focus compared with this new biting sharpness at the point of true focus. Perhaps Leica lenses, particularly those from the current ASPH range, are not an advantage in this respect as the in focus image is just so good.

[lars_bergquist]

Jamie, you did NOT read the article incorrectly. To repeat, aperture and subject "magnification" (actually, reproduction ratio, which is nearly always negative on the film/sensor) are the only variables that affect depth of field.

 

If you shoot a subject with a reproduction ratio you can control closely (maybe because one dimension is equal to one side of the image) with the same reproduction ratio and the same aperture, but with two lenses of different focal length, d.o.f. will be the same, provided of course that you do not change the reproduction ratio ex post facto by different print or screen sizes. BUT in order to achieve the identical reproduction ratio, you will have to change the camera-subject distance (closer with the wider lens) which will of course lead to a difference of perspective in the image. I think this is what confuses many people.

 

As always, when we have a complicated subject, some people delight in making it even more complicated. Every little obscurity is sacred, just like every little sperm in 'The meaning of life'. A sense of proportion is a precious thing. Not only in photography, but also when we discuss it.

 

The old man from the Age of Monty Python

[lars_bergquist]

Peter, you can always increase percieved depth of field by decreasing the difference between sharp and unsharp, i.e. making the sharp part less sharp! During the 1930's Leitz actually advertised the 73mm 1.9 Hektor as doing just that! (This reminds me of the immortal answer given to people who complain about their software: "It's not a bug, it's a feature.") And this is also essentially what a pinhole camera does.

 

The physical depth of the film emulsion, and also its light scattering properties, does indeed decrease maximum attainable sharpness to a value which can be lower than the lens can produce. This can lead to a different percieved depth of field. Also, when the current aspherical lenses were introduced, people complained that the transition from sharp to unsharp had become unpleasantly harsh. They did actually complain about too much sharpness, a complaint that has been heard for more than a hundred years now. Eugène Delacroix tells in his diary how his first sight of a daguerreotype made him physically sick by its plethora of senseless fine detail ... Contrary to what most people think, 'academic' or 'salon' painters worked with fairly broad brushes. Too much detail was considered kitschy.

 

The old man from the Age of the Box Camera

[Peter Branch]

......... To repeat, aperture and subject "magnification" (actually, reproduction ratio, which is nearly always negative on the film/sensor) are the only variables that affect depth of field........

 

Lars, I agree with you there is no benefit from making this topic any more complicated than it needs to be, and is already.

 

However for us purists the complexity starts when you start to define "aperture", that is where entry and exit pupils etc come into play, and every calculation of DoF uses some assumptions about visual acuity etc, so yes, for a given set of assumptions "aperture" and "reproduction ratio" are the key variables but that is not the whole story.

 

Why do digital images and particularly those from the M8 seem so much sharper at the point of critical focus? I'm not sure I'm fully understanding this aspect yet.

[lars_bergquist]

---------

However for us purists the complexity starts when you start to define "aperture", that is where entry and exit pupils etc come into play, and every calculation of DoF uses some assumptions about visual acuity etc, so yes, for a given set of assumptions "aperture" and "reproduction ratio" are the key variables but that is not the whole story.

 

Why do digital images and particularly those from the M8 seem so much sharper at the point of critical focus? I'm not sure I'm fully understanding this aspect yet.

 

Peter, it is true that for tele and retrofocus lenses, the position of the exit pupil does not coincide with the actual position of the iris diaphragm. This in theory does affect the amount of d.o.f. but I was interested in the principle, which did not seem to be well understood. There is also a lower limit to the size of the nits I choose to pick. The objection, If you will pardon me, does have a slightly Stephen Potteresque flavour ("Yes, but not in the south.")

 

I think that the extra sharpness is because of the absence of the definition-degrading depth and turbidity of the film emulsion. I have heard that opinion expressed by others, too.

 

The old man from the Age of the Wet Plate

[Jamie Roberts]

{snipped}

If you shoot a subject with a reproduction ratio you can control closely (maybe because one dimension is equal to one side of the image) with the same reproduction ratio and the same aperture, but with two lenses of different focal length, d.o.f. will be the same, provided of course that you do not change the reproduction ratio ex post facto by different print or screen sizes. BUT in order to achieve the identical reproduction ratio, you will have to change the camera-subject distance (closer with the wider lens) which will of course lead to a difference of perspective in the image. I think this is what confuses many people.

{snipped}

The old man from the Age of Monty Python

 

Lars (and Andy), thanks very much for clearing that up for me (I think ). I do believe I'm beginning to understand what you're saying, and how I was confused as well.

 

[

M: Look, I CAME HERE FOR AN ARGUMENT, I'm not going to just stand...!!

Q: OH, oh I'm sorry, but this is abuse.

M: Oh, I see, well, that explains it.

Q: Ah yes, you want room 12A, Just along the corridor.

]

[squarenegs]

Andy's explanations are excellent all round. My first two points are an amplification of what he said; then I'll show how his comments regarding the need for specification of format can be subsumed into a fuller use of the 'crop factor,' applying it to both aperture and focal length.

 

1) Regarding "field of view" in absolute terms: a 15mm on a 35mm camera is a super-wide, but a 15mm was the normal and only lens on a Minox (8x11 mm). Obviously, if you cut the center 8 x 11 mm segment out of a 35mm frame shot with a 15mm lens, it will be identical with the 8 x 11 mm image shot by a Minox. And if you blow up both to, say, 8x10, they will both show identical depth of field. But if you blow up a Minox shot to 8x10 and blow up a 24x36mm image from a 50mm lens ("normal" field of view in both cases), the Minox will show greater depth of field because it was made with a 15mm lens.

 

2) As you said, our depth of field markings are out of date. The DoF engravings on our lenses go back to Barnack's days. They're calculated for an enlargement of 4x6 using a far larger blur circle than we would use today, simply because lens technology is so much better.

 

But here's the interesting part:

3) Again, as Peter Karbe explained, when you start talking crop factor (which after all is exactly what we're talking about when we talk about 6x6 cm vs 24x36 mm vs 18x27 mm etc), the only way to do it fully is to multiply both the focal length and the aperture by the crop factor.

 

Crop factor is defined as the ratio of the diagonals of the two formats you are comparing. For example:

diagonal of 24 x 36 mm format = square root of (24^2 plus 36^2) = 43.27 mm

diagonal of 18 x 27 mm format = square root of (18^2 plus 27^2) = 32.45 mm

 

43.27 / 32.45 = 4/3

 

Thus, 4/3 (or 1.3333333...) is the crop factor of the M8 compared with 35mm film.

 

Now apply the crop factor to both focal length and aperture to define a lens exactly in regard to both field of view and depth of field:

50/1 becomes "equivalent to" a 67/1.3

28/2 becomes "equivalent to" a 37/2.7

12/5.6 becomes "equivalent to" a 16/7.5

 

In other words, applying the crop factor to both focal length and aperture does the same thing as specifying focal length, crop factor, and format covered.

 

Second example: 15mm lens on Minox as compared to 35mm:

diagonal of 8x11 frame is 13.6 mm, so "crop" or "magnification" factor compared to full-frame is 3.18.

 

Thus, the 15/3.5 lens of the Minox is (in depth of field and field of view) "equivalent to" a 48/11 on full frame.

 

Carrying out the comparison this way makes it immediately obvious why the Minox shows so much more depth of field than a 'normal' on 35mm: Set a 50mm lens to f/11 on a full-frame camera, and you'll have the same depth of field as you have with the Minox 15/3.5.

 

Third example: 7mm f/2 lens on Digilux 2 compared to 35mm:

diagonal of D2 frame is 11 mm, so 'crop factor' is 3.9 compared to full frame. Thus, the 7/2 lens of the D2 is "equivalent to" a 28/8 on full frame. Thus at its widest, the D2 approximates the field of view of a 28mm on full frame; and wide open at that 7mm focal length, it has the same depth of field as a 28mm lens at f/8 on full frame.

 

See the article in LFI that I mentioned above for a very strenuous proof of the idea (or the LuLa debate link for a passing nod to the relationship, as if it were well known among photographers ).

 

In my opinion, Peter Karbe deserves great credit for discovering (or at least for explaining) this relationship.

 

--HC

Howard:

 

Geez... your the only person I can understand on this thread! Also, you are correct.

There is a lot of confusion out there. Bottom line... a given focal length lens has the same characteristics regardless of what kind or size of capture medium it projects an image onto. Just let's not get into that thing about DOF actually being the same for any focal length if the images projected are of equal size. ...Or has it already been mentioned??? ;-)

 

All the best to everyone

on the Form,

 

Sam

[ho_co]

Oh, my Lord, Sam! You've lost all your standing by agreeing with my kerfluffel. Thanks for the support!

 

I hate to resurrect this thread, but I found the article on the uselessness of trying to use 'standard' depth of field arguments with digital imagery. I mentioned it above but couldn't immediately find it on the web. Gary Ferguson does a simple and practical photographic demo that will knock your socks off if you haven't seen it, and offers some workarounds. See Digital Focusing Part One.

 

Also, if you're still not convinced about applying crop factor to aperture when comparing lenses, see the exquisite article on the Mamiya 645ZD by Efrain Garza and Rubén Osuna (I don't think it was mentioned previously in this thread) at Mamiya ZD A 35mm-like Medium Format Camera.

 

I hope I haven't repeated things already covered.

 

Respectfully,

--HC

[gibbons]

I know this is a late post; however, Leica M8 manual mentions "The extension factor." (pg 87, English) The following maybe useful to people.

 

The nominal focal lengths of Leica M lenses are based

on the 35mm-format, i.e. on a film format of

24x36mm. In comparison, with its 18x27mm, the

sensor in the LEICA M8 is somewhat smaller though –

by a factor of 0.75. Therefore, when used on the

LEICA M8, these lenses have angles of view corre-

sponding to lenses with focal lengths that are longer

by a factor of 1.33 (1.33 = reciprocal of 0.75). This

has the respective effects on their perspective, but

not on their depth of field, which, with the LEICA M8,

can also be read directly off the lens (see the lens

instructions for more details).

[2Bébèrt]

Gibbons,

 

of course I have read the Leica manual to before starting this thread but when I

use the lens-markings in the field, the DOF is not correct. The -1 diaf-rule is

a good workaround in daily street photographing.

[ho_co]

And, Jeremy, it was Leica's Director of Lens Development, Peter Karbe, who to my knowledge first showed that the instruction manual is in error. See, again, LFI 3/2006, pp 40-47.

 

--HC

[gibbons]

Interesting about LFI; however, I don't have the issue you're referring to, but I'll try to hunt it down.

 

If this issue has been addressed why hasn't Leica offered an official response, addendum or supplement to the manual? Mind you I just bought my camera about two months ago which should have been enough time for the company to update the manual before shipping (assuming my camera was made and shipped only months before I bought it) or at the very least post a softcopy addendum. If this is a "known" issue with a general work around it should be updated formally in documentation (imho, LFI doesn't count as a formal Leica documentation).

 

Perhaps this is Leica's mode of operation, and LFI is considered a "formal" addendum, but I doubt it. If there is another formal reference beyond LFI please send the link. I don't mean to be a doubting thomas, but it's the engineer in me

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